Micro-Electro-Mechanical Systems, or MEMS can be defined as miniaturized mechanical and electro-mechanical systems where at least some elements have some mechanical functionality. Since MEMS devices are created with the same tools used to create integrated circuits, micromachines and microelectronics can be fabricated on the same piece of silicon to enable machines with intelligence.
MEMS structures can be applied to quickly and accurately detect very small changes in physical properties. For example, a microelectromechanical gyroscope can be applied to quickly and accurately detect very small angular displacements. Motion has six degrees of freedom: translations in three orthogonal directions and rotations around three orthogonal axes. The latter three may be measured by an angular rate sensor, also known as a gyroscope. MEMS gyroscopes use the Coriolis Effect to measure the angular rate. When a mass is moving in one direction and rotational angular velocity is applied, the mass experiences a force in orthogonal direction as a result of the Coriolis force. The resulting physical displacement caused by the Coriolis force may then be read from a capacitively or piezoresistively sensing structure.
In MEMS gyros the primary motion cannot be continuous rotation as in conventional ones due to a lack of adequate bearings. Instead, mechanical oscillation may be used as the primary motion. When an oscillating gyroscope is subjected to an angular motion orthogonal to the direction of the primary motion, an undulating Coriolis force results. This creates a secondary oscillation orthogonal to the primary motion and to the axis of the angular motion, and at the frequency of the primary oscillation. The amplitude of this coupled oscillation can be used as the measure of the angular rate.
The challenges in MEMS gyroscopes are related to the fact that the magnitude of the sense-mode response amplitude is extremely small. In implementations, fabrication imperfections result in non-ideal geometries in the gyroscope structure and cause the drive oscillation to partially couple into the sense mode. Understanding the relative magnitudes of the drive and sense mode oscillations, even smallest undesired couplings from the primary oscillation could exceed the sensed Coriolis response. The Coriolis force is, however, proportional to the drive velocity of the mass and the coupled force to the position of the mass, so there is always a π/2 phase difference between the Coriolis response and the mechanical force. The quadrature signal can therefore be relatively easily separated from the Coriolis signal during amplitude demodulation at the drive frequency. However, existing configurations tend to increase the size and complexity of the sensing device configurations. A further implication of the large relative magnitude of the quadrature signal is that the stability of quadrature over temperature and over time is important. If the part of the quadrature signal that mixes into the rate signal varies significantly, the stability of the gyroscope deteriorates.